This invention relates to coupling/decoupling capacitor multipliers and, in particular, to a capacitor multiplier employing op-amp technology.
Every real capacitor can be represented by an equivalent circuit, in which:
C=equivalent capacitance; PA1 ESR=equivalent series resistance: PA1 RL=leakage resistance; and PA1 LS=series inductance. PA1 RL is in parallel with series connected ESR and C which circuit is in series with LS PA1 IL represents leakage current; PA1 Ceq represents equivalent capacitance; PA1 ESR represents the equivalent series resistance; and PA1 Ios and Vos represent offset current and offset voltage, respectively. PA1 1. Substantial losses due to relatively large value of R3 occur. Consequently, high Q capacitance cannot be realized. PA1 2. Leakage current can be very substantial for op-amp with large offset voltage or current. Also, it will change with temperature as offset current and voltage does. PA1 3. Maximum DC operating voltage is limited to the maximum operating voltage of an op-amp. PA1 4. Only a grounded capacitor can be implemented. Consequently, the configuration described above cannot be used as a floating capacitor, ruling out a coupling capacitor application.
In all low and medium frequency applications, the series inductance can be ignored and the capacitor's properties can be sufficiently described by RL, ESR, and C. Commonly, instead of leakage resistance RL, a leakage current IL is used to describe leakage of the capacitor. An ideal capacitor should have both IL and ESR equal to zero.
In many applications there is a need for large capacitors with low ESR, low leakage, and high operating voltage. The prior art has addressed the need for large capacitors in the following way.
To achieve a large capacitor, large electrolytic or tantalum capacitors are employed. Such a capacitor suffers from leakage current and usually substantial AC losses. In addition, the size of such capacitors are proportional to the product of capacitance and square of operating voltage, leading to unusually large sizes for high operating voltage.
Another approach to achieve a large capacitor is found in the use of a capacitor multiplier which employs operational amplifier technology. An example of such a solution is shown in FIGS. 1, 2 and 3. The capacitor multiplier of FIG. 1 creates an equivalent capacitor with the following parameters ##EQU1## in which: R1, R2, R3, and C1 are components used with the op-amp multiplier in the circuit;
The shortcomings of the capacitor multiplier in FIG. 1 are summarized as follows:
The capacitor multiplier of FIG. 2 creates a negative capacitor not used in coupling or decoupling applications. Furthermore, the shortcomings described in Paragraphs 2, 3, and 4, above for the capacitor multiplier of FIG. 1 also apply to the multiplier shown in FIG. 2.
The capacitor multiplier configuration shown in FIG. 3 creates an equivalent capacitor as described by the following equation: EQU Ceq=C1.multidot.(R2/R1+1)
While the capacitor multiplier shown in FIG. 3 solves the leakage problem of the circuits shown in FIGS. 1 and 2 it, however, experiences the shortcomings of paragraphs 3 and 4 listed above for the multiplier of FIG. 1.